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This Building Block contains a detailed overview of the habitat requirements for salt marshes. The building block provides a guideline to check whether a certain location is suitable or can be made suitable for the establishment of salt marshes. Through an overview of conditions and a flow chart in the tab 'how to use', planners and designers can determine whether salt marshes are a promising option for incorporation into a design. This Building Block is mainly applicable in the initiation phase, its focus is on physical conditions. Practical experience with the design (conditions) of salt marshes is gained in the projects of Harlingen and Delfzijl (The Netherlands). The lessons learned in these projects considering the habitat requirements are described under the tab ‘Practical applications’.
Natural salt marsh on the island of Texel, The Netherlands (Alma de Groot, Wageningen Marine Research)
- The case description page Mud motor Wadden Sea (follows) gives information about the project case of Koehoal, Harlingen. In this case dredged sediment is used to increase the availability of silty sediments and thereby enhance salt marsh development.
- The case description page Saltmarsh development Marconi, Delfzijl (follows), gives general information about the project case in which a salt marsh base is built and combined with measures that improve the boundary conditions for further salt marsh development. The salt marsh base is completed with testing zones for both planted vegetation and natural development of pioneer vegetation.
- With this building block, anyone having a basic level of knowledge and/or working experience in temperate coastal systems can make a first-order assessment of the suitability for salt marshes on a specific location. To fully assess the suitability for salt marshes one should have additional morphological and ecological expertise on salt marshes.
- The two main added values of a salt marsh to Building with Nature (BwN) type of projects are:
- Enabling the realisation of natural barriers for coastal defence due to wave reduction.
- Enabling adaptation to sea level rise.
- Salt marshes provide several additional ecosystem services as improvement of the water quality, biodiversity, rest and foraging places for migrating birds, carbon storage, wave reduction, sediment management (beneficial use of dredged material), silt agriculture, limitation of bank erosion and a recreational value due to its attractive landscape.
Determination flow chart
This flow chart can be used to determine the suitability of a site for marsh to develop. At the end of the flow chart we assume establishment of pioneer vegetation (Spartina anglica and/or Salicornia spp.) with the proper environmental conditions for a sustainable marsh to develop. The set-up of the flow chart and the habitat requirements for salt marshes are discussed below. The colours in the flow chart correspond to the colours as used for the spheres under the section on 'habitat requirements'.
Habitat requirements for salt marshes
Salt marshes are ecosystems that are vegetated by halophytic plants and that are regularly flooded by the sea. The salt marshes discussed here are located in a temperate climate, at the upper part of the intertidal zone. They extend vertically from well below Mean High Tide up to the yearly highest water levels.
The tidal flow brings in fine-grained suspended sediment. Eco-engineering plants trap this sediment, resulting in a marsh surface that steadily grows upwards. This is called marsh accretion. Typical marsh accretion rates are in the order of a centimeter per year, depending on tidal range, soil stability by the local vegetation and the availability of suspended sediment. Marsh accretion rates need to be equal or preferably higher than current rates of enhanced sea-level rise to ensure a long-term sustainable salt marsh.
Several marsh types can be found in estuaries. Salt marshes, freshwater marshes and peat-based marshes.
- Salt marshes are inundated by saline water and have an average salinity greater than 0.5 g of solutes per kg of water (Odum 1988). Ample availability of sediments results in marsh accretion of these salt marshes.
- In areas with little sediment in the water, the soil consists of peat produced by the salt-marsh vegetation (Allen 2000).Here, marsh accretion is mainly promoted by the build-up of organic material from the local vegetation, resulting in peat-based marshes.
- Sometimes estuaries harbour comparable ecosystems but with a freshwater vegetation instead of salt-tolerant plants. In that case we deal with freshwater marshes.
Important habitat requirements handled in this building block may differ for peat-based marshes and freshwater marshes. In this Building Block focus is on salt marshes that are inundated by saline water. Furthermore, this building block describes the habitat requirements for a salt marsh ecosystem, without a description of specific plant species. Depending on the geographic location of the marsh and salinity conditions, different species will establish/settle. Different developmental stages can be distinguished in time, with a succession from pioneer to climax salt marsh. For more information on plant species in temperate salt marshes, one is referred to http://www.waddensea-secretariat.org/monitoring-tmap/manual-guidelines and to table of species.
The Trilateral Monitoring and Assessment Programme (TMAP) is the common monitoring programme for the Wadden Sea. In this programme three main salt marsh zones are distinguished: the pioneer zone where plant growth starts at about 40 cm below mean high tide (MHT); the low marsh, inundated during mean spring tides (100-400 floods/year), and the middle/high marsh with less than 100 floods per year. In addition, the sandy green beach and the brackish marsh can be differentiated by a special type of vegetation. Adjacent to the salt marshes, fresh (anthropogenic) grassland occurs.
Salt marshes have high nature values and are protected under European and national laws. Due to their relatively high elevation within the tidal frame, they reduce wave height during storms. As a result, salt marshes play an important role in contributing to coastal protection.
The description of salt marsh habitat requirements is based on the 4-spheres approach: biosphere, hydrosphere, lithosphere and atmosphere. They all interact with each other and cannot be evaluated on their own. For each sphere, relevant salt marsh parameters are described. The limit values are provided if they are known and generally valid and applicable. These values are derived from literature. A complete overview of the four spheres and the parameters relevant to salt marshes is shown in the habitat requirement tree below. A description of each of the spheres can be accessed by clicking the 'sphere-balloons'.
Tip:select a 'sphere' in below diagram for more detailed information.
Two practical examples are presented, Mud Motor Wadden Sea, Harlingen and Salt Marsh development, Marconi, Delfzijl. Both Delfzijl and Harlingen are harbour cities situated in the north of the Netherlands.
Harlingen (left) borders the Wadden Sea area. The Wadden Sea is a coastal sea protected from the North Sea by barrier islands. It is home to the largest unbroken system of intertidal sand and mud flats in the world. Since 2009 it serves as UNESCO world heritage site. (http://whc.unesco.org/en/list/1314/).
Delfzijl (right) is situated along the Ems-Dollard estuary. This estuary connects the Ems river to the Wadden Sea. The Ems-Dollard estuary is one of the last two open estuaries in the Netherlands.
Both Wadden Sea and Ems-Dollard estuary are of significant ecological importance for estuarine and intertidal flora and fauna. Click on the area maps for a project description.
Allen JRL (2000) Morphodynamics of holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. Quaternary Science Reviews 19: 1155-1231
Bakker JP, Bunje J, Dijkema K, Frikke J, Hecker N, Kers B, Körber P, Kohlus J & Stock M (2004). Chapter 7: Salt marshes. In: Essink K, Dettman C, Farke H, Laursen K, Lüerβen G, Marencic H & Wiersinga W (Eds.). The Wadden Sea Quality Report 2004. Wadden Sea Ecosystems No. 19-2005. Trilateral monitoring and assessment Group, Commen Wadden Sea Secretariat, Wilhelmshaven, Germany. http://www.waddensea-secretariat.org/sites/default/files/downloads/08-saltmarshes-10-09-21_0.pdf
Cahoon DR (2006) A review of major storm impacts on coastal wetland elevations. Estuaries and Coasts 29: 889-898.
Ford H, Garbutt A, Ladd C, Malarkey J, Skov MW (2016) Soil stabilization linked to plant diversity and environmental context in coastal wetlands. Journal of vegetation Science 27: 259-268.
Odum WE (1988) Comparative ecology of tidal freshwater and salt marshes. Annual Review of Ecology and Systematics 19: 147-176.
Olff H, De Leeuw J, Bakker JP, Platerink RJ, Van Wijnen HJ, De Munck W (1997) Vegetation succession and herbivory in a salt marsh: changes induced by sea-level rise and silt deposition along an elevational gradient. Journal of Ecology 85: 799-814.
Huiskes AHL, Stienstra AW, Koutstaal BP, Markusse MM, Van Soelen J (1985) Germination ecology of Salicornia dolichosachya and Salicornia brachystachya. Acta Botanica Neerlandica 34: 369-380.
Kirwan ML, Guntensperger GR, D’Alpaos A, Morris JT, Mudd SM & Temmerman S (2010) Limits on the adaptability of coastal marshes to rising sea level. Geophysical Research Letters 37, L23401.
Temmerman S, Govers G, Meire P & Wartel S (2003) Long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium. Marine Geology 193: 151-169
Temmerman S, Govers G, Meire P, Wartel S (2004) Simulating the long-term development of levee-basin topogrpahy on tidal marshes. Geomorphology 63:39-55.
Temmerman S, Bouma TJ, Van de Koppel J, Van der Wal DD, De Vries MB, Herman PMJ (2007) Vegetation causes channel erosion in a tidal landscape. Geology 35: 631-634
Van Duin WE, Dijkema KS (2012) Randvoorwaarden voor kwelderontwikkeling in de Waddenzee en aanzet voor een kwelderkansenkaart. Rapport C076/12, IMARES, Wageningen University & Research. http://edepot.wur.nl/220052
Van Loon-Steensma JM, De Groot AV, Van Duin WE, Van Wesenbeeck BK, Smale AJ (2012) Zoekkaart kwelders en waterveiligheid waddengebied. Alterra Rapport 2391, Alterra, Wageningen University & Research. http://edepot.wur.nl/244770